When burning fossil fuels to produce energy, one typically uses a high temperature combustion process in the presence of air. Unfortunately, this type of process produces both nitrogen oxides (NOx), which are well-known pollutants, and other components that are harmful to health or the environment, such as carbon monoxide and unburned hydrocarbons. Thus, it is important to remove these materials prior to their release into the environment.
There have been many investigations into methods that allow for the removal of these substances. Two methods that are known are combustion modifications and adsorption techniques. Unfortunately, each of these has its disadvantage. The former allows for only limited maximum removal of NOx, and the latter has limited capacity.
A third method for addressing the problem of noxious exhaust gases is catalytic removal, which by comparison, is extremely effective in removing large proportions of unwanted exhaust components and is capable of treating very large volumes of exit gases for long periods of time. In order to effect the reduction of NOx in exhaust gases through catalytic reduction processes, it is necessary either to introduce a reducing agent, such as ammonia, and/or to use the unburned hydrocarbons present in the waste gas effluent.
The selective catalytic reduction (SCR) process consists of the reduction of NOx, (NO, N2O and NO2) species using ammonia as a reductant in the presence of oxygen and a catalyst to produce nitrogen and water. The SCR process is widely used in the U.S., Japan, and Europe to reduce emissions of large utility boilers and other commercial applications. Increasingly, SCR processes are being used to reduce emissions in mobile applications such as in large diesel engines like those found on ships, diesel locomotives, automobiles and the like.
Various catalysts have been used in SCR processes. Initial catalysts, which employed platinum or platinum group metals, were found unsatisfactory because of the need to operate in a temperature range in which explosive ammonium nitrate forms. In response to environmental regulations in Japan, the first vanadium/titanium SCR catalyst was developed, which has proven to be highly successful. Further development has resulted in the development of vanadium catalyst deposited on titanium oxide/tungsten oxide support material.
Although vanadium catalysts supported on titanium oxide/tungsten oxide provide excellent performance, the potential toxicity of vanadium oxide has led to health concerns regarding the use of SCR catalysts for mobile applications. Presently, there are no alternatives that rival the performance of the high performance vanadium pentoxide catalysts supported on tungsten oxide/titanium oxide.
One alternative catalyst technology being proposed are transition metals supported on zeolites such as those under the trade name ZSM-5™ by Exxon-Mobile. Such catalysts are described, for example, in U.S. Patent Publication No. US 2006/0029355, European Patent Application Publication No. EP 299294 A2, European Patent No. EP 738179 B1 and International Application Publication No. WO 2004/022229 A1. However, this technology is limited by the high cost of zeolite catalysts, which can be a factor of 10 more expensive than comparable titania-supported catalysts.
A number of publications describe various mixed oxide catalysts systems as NOx reduction catalysts. For example, U.S. Pat. No. 3,279,884 to Nennenmacher et al. describes the removal of NOx in an oxygen containing stream over catalytic metal oxides of V2O5, WO3, MoO3 or their mixtures.
U.S. Pat. No. 4,048,112 to Matsushita et al., describes the use of vanadia on anatase titania as effective NOx removal catalysts.
U.S. Pat. No. 4,085,193 to Nakajima et al., describes improved performance of NOx catalysts by supporting V2O5, WO3, MoO3 or their mixtures on titanium dioxide.
U.S. Pat. No. 4,221,768 to Inoue et al. describes NOx catalysts comprising mixed oxides of TiO2—SiO2 or TiO2—ZrO2—SiO2, and identifies the innate acidity of the metal oxide support. The patent also describes the use of Mn, Ce, Fe, Zr, Si and Ti in DeNOx catalyst compositions.
U.S. Pat. No. 4,833,113 to Imanari et al. describes an improved DeNOx catalyst comprising an oxide of titanium, an oxide of tungsten and an oxide of vanadium having support with a surface area of 80-200 m2/g and a pore volume of 0.1 to 0.5 mL/g.
Japanese Patent Publication JP 2003/093880 to Hirakawa et al., describes a catalyst comprising a composite oxide obtained by neutralizing a soluble titanium compound, a soluble silicon compound and further adding a soluble tungsten compound and at least one oxide of vanadium, molybdenum and tungsten.
U.S. Patent Application Publication No. 2006/0084569 to Augustine et al., describes a high activity DeNOx catalyst prepared by deposing vanadium oxide on a titania-supported metal oxide, such as tungsten oxide, where the supported metal oxide has an isoelectric point at a pH of less than or equal to 3.75 prior to depositing the vanadium.
U.S. Pat. No. to Suda et al. describes a titania-zirconia powder where at least part of the zirconia is dissolved in the titania crystalline phase or at least part of the titania solid is dissolved in the zirconia crystalline phase. Also described is a titania-zirconia-alumina powder.
U.S. Pat. No. 5,021,392 to Daly et al, describes binary oxidic catalyst support materials comprising titania and zirconia prepared by a pH swing technique or a constant pH technique followed by calcination below 450° C.
U.S. Pat. No. 7,247,283 to Hedouin describes a mixed zirconium-titanium oxide comprising between 30% to 40% by weight titanium oxide and either a pure ZrTiO4 or a mixture of phases of ZrTiO4 and anatase. produced by thermal hydrolysis of a zirconium compound and a titanium compound.
Japanese Patent Publication JP2006068663 describes a catalyst for the treatment of exhaust gas that contains a Ti—Si composite oxide and/or a Ti—Zr composite oxide and an oxide of manganese. The publication also describes that the catalyst may also comprise an oxide of copper, chromium, iron, vanadium tungsten, nickel or molybdenum.
U.S. Pat. No. 4,855,115 to Imanari et al. describes a DeNOx catalyst comprising an oxide of titanium, and oxide of at least one of tungsten and molybdenum, an oxide of vanadium and an oxide and/or a sulfate of at least one of yttrium, lanthanum, cerium, neodymium. The patent also describes a catalyst that comprises a metal selected from yttrium, lanthanum, cerium neodymium, copper, cobalt, manganese and iron deposited on zeolite.
A common SCR catalyst support sold under the trade name DT52™ by Millennium Inorganic Chemicals, Inc. contains tungsten oxide deposited on titanium oxide. It requires the further addition of vanadium pentoxide to become a functioning catalyst having excellent activity which has been a standard catalyst for SCR processes since its introduction in the late 1980's.
Despite the various mixed oxide catalysts being developed and the zeolite supported catalysts, there exists a need for low cost vanadium-free catalysts that provide high catalytic activity in the SCR reaction.